The hydraulic-chemical bonding and refractory features of the magnesia cement according to the present invention are best understood when compared with state of the art cements. Known cements such as Portland cements or calcium aluminate cements and their numerous modifications are typical hydraulic bonding agents, or hydraulic binders. They set and cure as a result of chemical reactions of cement clinker with water and the formation of main bonding phases like calcium silicate hydrates, ettringite, brownmillerite and calcium aluminate hydrates, respectively. The calcium aluminate cements are mainly used for refractories and ceramics. However, the lime content of calcium aluminate cements lowers the refractoriness of alumina or basic refractory products. Limefree hydraulic cements, so called hydratable alumina, were developed and are used for corundum, alumina spinel, aluminum silicates and less often for basic refractories.
In contrast to hydraulic cements used for low temperature applications, there are several requirements regarding effective refractory and ceramic cements. The most important features are:                bonding not only at ambient temperature but also in the temperature range close to the temperature where the formation of ceramic bonds occurs;        high purity or contamination with only such constituents which do not decrease the refractoriness or high temperature strength;        low and non-toxic emissions during heat-up;        a phase composition assuring volume stability during cyclic heating and cooling.        
Other known bonding agents used among others for refractories are primarily composed of soluble silicates, silica fume, phosphates and organic polymers or resins. Such bonding agents may comprise magnesia as an accelerator or even as the main component of the matrix and/or of the aggregates.
Alkali silicates, i.e. water glass, are another group of chemical binders for ceramics and refractories. Unfortunately, alkali silicate bonded products have only a minor refractoriness and resistance to chemical wear by e.g. slags, and are therefore mainly used for gunning or repair mixes but not for high temperature permanent or wear lining.
Phosphates are widely used as chemical bonding agents for basic and alumina refractories and ceramics. Hardening at ambient temperature is, however, often to slow, thus application of enhanced temperature or admixture of additives, e.g. one or more hydraulic bonding agents, is usually necessary. Also, the refractoriness of phosphate bonded products is not always satisfactory. Additionally, the phosphate content of the refractory lining of metallurgical furnaces can negatively influence the purity of products such as, for example, steel.
The hydraulic properties of lime and magnesia at low temperature, particularly of low particle size and/or calcined lime or magnesia, are generally known from textbooks and encyclopedia editions. Neither of the two compounds may be used alone as hydraulic cement because of the following reasons:                Calcium hydroxide, the product of lime hydration, shows certain solubility and would dissolve in an excess of water leading to a decrease of strength.        Very finely grinded magnesia or magnesia calcined at low temperature, usually obtained by calcination of magnesium hydroxide or magnesium carbonate, would hydrate very rapidly forming almost insoluble magnesium hydroxide, so called brucite. An uncontrolled setting would further result in deterioration of the mechanical properties of the end-product. Hydration of magnesia is significantly increased by the free lime content of MgO. One of the substantial issues of magnesia bonding according to the reaction MgO+H2O=Mg(OH)2 is a significant increase in volume, typically of about 45-55%, frequently of about 50 to 51%, resulting in a tendency for cracking and other defects during setting and drying. This property can especially be detrimental for magnesia cement-bonded basic refractories because magnesium hydroxide formed during cement hydration can act as brucite nuclei accelerating hydration of the magnesia matrix and aggregates. The decomposition of brucite during heat-up, accompanied by a usually 45-55%, typically 50 to 51% decrease in volume, would be a substantial drawback of pure magnesia cement-bonded refractories and ceramics.        
The most commonly used magnesia cements are so-called Sorel cements which are based on reactive magnesia and magnesium salts such as sulphates and/or chlorides and which form the bonding matrix via magnesium oxychlorides and/or oxysulphates. Patent application US2005103235 describes properties and drawbacks of Sorel cements. Sorel cements are, however, less suitable for refractory and high temperature applications, due to a loss of strength and harmful emissions during heat-up.
The cements disclosed in US2005103235 have not been proved to be suitable for refractory applications, the nature of bonding is exclusively hydraulic and the main bonding species is brucite. Moreover, fluxes and mineralizing agents known for decreasing refractoriness are used for calcination, and grained reactive magnesia is the basic component of the cements. For that reason setting accelerators are necessary to achieve an acceptable setting time.
DE1471297 discloses and claims a refractory material which contains a cementitious mixture of non-plastic, fine magnesia and 0.1 to 15% of an aliphatic hydroxy tricarboxylic acid, preferably citric acid, or a salt thereof. However, it is known in the art that using magnesia of high specific surface area together with a well soluble, strongly complexing organic acid such as citric, would—after mixing with water—cause substantial difficulties in setting time adjustment and would deteriorate the rheological and the final mechanical properties of the end-product.
Similarly, U.S. Pat. No. 3,923,534 discloses a cold-settingrefractory composition comprising magnesia of low reactivity with a surface are of less than 2 m2/g and the anions of a carboxylic acid such as citric acid being a constituent of a water-soluble aluminium phosphate complex binding agent. However, as mentioned hereinbefore, the use of a well water-soluble organic acid such as citric acid would entail various undesired side-effects. Some disadvantages of phosphate binding agents for refractories have already been outlined before.
EP1953487 discloses a fire-resistant material consisting of known refractory raw materials, among others dead-burned and fused magnesia, bounded with a heatactivated binder being a carboxylic acid, especially hydroxy-carboxylic acid, or a mixture of different carboxylic acids. The disclosed fire-resistant material used for lining metallurgical vessels is, however, not a hydraulic binder which can set and harden after mixing with water at ambient temperature.
Another cementitious refractory composition comprising fine magnesia and 0.25 to 5% of a boron compound and a soluble chromium compound in a ratio of from 3:1 to 1:1.5, calculated as CrO3 and B2O3, respectively, is known from GB723924. However, the use of chromic acid and soluble chromates for refractories is presently forbidden, because of alleged carcinogenic properties of Cr(VI) compounds.
U.S. Pat. No. 3,751,571 describes a refractory lining for coreless induction furnaces formed from castable refractory cement. The term cement as used therein relates to a cement composition consisting of refractory aggregates and bonding components comprising reactive magnesia and a small amount of an organic acid, i.e. oxalic acid. The oxalic acid used in example I is, however, readily soluble and forms sparingly soluble magnesium salts. From the disclosure of U.S. Pat. No. 3,751,571 it can be inferred that only a weak bonding of the cement was desired and in a certain temperature range, i.e. in the soft friable zone, this bonding completely disappeared.
JPS55150972 provides a cement for grinding stones for dental applications. Powders of magnesium oxide and an aqueous solution of a polyacrylic acid in the ratio of 3:1 are the main constituents of the cement. The polyacrylic acid forms chelate bonds with the molecules bridged by magnesium and the cement sets providing a high mechanical strength. This cement is not suitable, however, for high temperature applications because of the decomposition of polyacrylic acid at such temperatures.